Between January 1, 1981, and December 31, 1982, the Colorado Department of Health received reports of 1,185 culture-confirmed cases of Campylobacter jejuni infection. Incidence rates were highest among infants less than 1 year old and among persons aged 20-29 years. The distribution of cases by sex showed a predominance among males at all ages except 40-59 years, the most marked predominance occurring in infants under 1 year. The higher rates for males were also significant for all ages combined, for ages 10-19 years, and for ages 5-9 years. Neither Salmonella nor Shigella infections reported in Colorado during the same period showed the preponderance among males found for C. jejuni infections. Giardia infections, however, showed a weak male predominance, especially among children less than 10 years old. The preponderance of C. jejuni cases among males disclosed by this study was remarkable. The reasons for this phenomenon are not clear and need further research.